An ICD is an implantable medical device that detects atrial fibrillation (AF) or ventricular fibrillation (VF) within the heart of a patient then delivers one or more high-voltage electrical shocks using a set of leads having electrodes implanted in the heart in an attempt to terminate such fibrillation and revert the heart to a normal sinus rhythm. Fibrillation pertains to the chaotic beating of the chambers of the heart. During fibrillation, there is little or no net blood flow in or out of the fibrillating chambers. AF (i.e. fibrillation occurring within the atrial chambers of the heart) is not typically life-threatening, though it can sometimes trigger VF. VF (i.e. fibrillation occurring within the ventricular chambers of heart) is fatal if not terminated. VF is treated by automatically delivering one or more high-voltage defibrillation shocks directly to the ventricles of the heart using the ICD. State-of-the-art ICDs typically deliver defibrillation shocks at voltages in the range of 100 to 500 volts (V).
It is important to know the impedance of a defibrillation system. Following device implementation, typically, one or more high-voltage test shocks are delivered to the heart of the patient using the ICD and the resulting defibrillation impedance is measured and recorded. A physician examines the impedance values to verify that they are within an acceptable range. Abnormal defibrillation impedance values can indicate, for example, that the leads are defective or were not properly implanted. However, even if the defibrillation impedance values are initially within an acceptable range, defibrillation impedance can change during the months or years following implantation. Shifts in defibrillation impedance can indicate lead movement or lead fracture, fibrosis, or even electrolyte problems within the heart. A significant change in defibrillation impedance can reduce the effectiveness of defibrillation shocks. Lead fracture, indicated by a very high impedance value, typically results in a complete failure to deliver an adequate defibrillation shock.
State-of-the-art ICDs are capable of performing an automatic defibrillation impedance test to verify that impedance remains within acceptable range. Typically, a low-voltage pulse between five and ten volts is periodically delivered to the heart and the resulting impedance is measured to verify that it is within an acceptable range. Low-voltage pulses (i.e. less than 10 volts) are typically employed during the test so as not to cause pain within the patient or to unduly deplete the power supply of the ICD. However, for a variety of reasons, defibrillation impedance measurements based on low-voltage pulses are extremely inaccurate, i.e. the measurements are usually not indicative of the impedance that would actually arise if a high-voltage defibrillation shock were delivered. For example, low-voltages create only low current densities around the electrodes. These low current densities do not recruit very many ionic species and hence have a very high-voltage drop associated therewith. Thus, impedance measurements are non-linear with voltage. Other problems arise because red blood cells are insulators at low frequencies but become conductors at high frequencies. Thus, depending upon the spectral content of the test pulse as compared with the spectral content of the actual high-voltage defibrillation shock, there may be significant impedance measurement errors. As a result, the typical defibrillation impedance detection function provided within an ICD is only capable, at best, of reliably detecting lead fracture and is not typically useful for detecting other variations in defibrillation impedance, which may reduce the effectiveness of subsequent defibrillation shocks.
Accordingly, it would be highly desirable to provide improved techniques for estimating defibrillation impedance using an ICD or other implantable medical device and is to that end that the invention is primarily directed.